1. Field of the Invention
The present invention relates to a relatively small size fuel cell system for use in a portable appliance and a control method thereof.
2. Description of the Related Art
A direct methanol fuel cell (DMFC) has been developed progressively as a small power supply for a portable appliance from viewpoints that such an auxiliary device as a carburetor and a humidifier is not required, methanol is easier to handle than gas fuel such as hydrogen, and that it can be operated under low temperatures.
The DMFC has an electrode (MEA), anode electrode and cathode electrode, and there exist an active type in which the auxiliary device is used to supply fuel to the anode electrode and cathode electrode and a passive type in which natural power is used for that purpose. Although the former active type DMFC can raise the output density of the cell, such an auxiliary device as a pump for supplying fuel to the anode electrode and the cathode electrode is indispensable.
The DMFC needs methanol and water of the same mol for electrode reaction of the anode electrode. When high concentration methanol is supplied directly to the electrode, methanol crossover increases thereby dropping fuel usage efficiency. Then, to recover water generated by cathode electrode reaction efficiently and mix the recovered water with high concentration methanol within a mixing tank, a water recovery mechanism can be provided on the cathode electrode side as an auxiliary device. To block the volume of the apparatus from being increased due to installation of the water recovery mechanism on the cathode electrode side, it is possible to construct a system which circulates only fuel on the anode side without recovery of water on the cathode side.
In such a circulation type fuel cell system, it is important to maintain the concentration of fuel to be supplied to the anode electrode appropriately and maintain the amount of fuel within the mixing tank which feeds the fuel to the anode electrode appropriately. As the system for controlling the concentration and amount of fuel, various proposals have been made conventionally. For example, JP-A 2005-32610 (KOKAI) has proposed a system for controlling the concentration of mixed liquid by controlling the flow rate of fuel from the high concentration fuel tank and a cathode water recovery unit. Further, JP-A 2005-11633 (KOKAI) has proposed a system for controlling the concentration of mixed liquid by controlling the supply amount of fuel from the high concentration fuel tank and the water recovery amount on the cathode side. JP-A PH5-258760 (KOKAI) has proposed a system for controlling the amount and concentration of the mixed liquid by controlling the supply amount of fuel from the high concentration fuel tank and the supply amount of water from a water tank. JP-A 2005-108713 (KOKAI) has proposed a system which feeds air discharged from the cathode electrode to a condensation unit and operates a condensation fan and water recovery unit to change the amount of water to be supplied to a mixing tank, thereby controlling the concentration and amount of the mixed liquid.
The conventional fuel cell system includes a water recovery unit on the cathode side, a fuel supplying unit on the anode side, a water recovery circulation path which connects the cathode side with the anode side, and a water tank for supplying water from outside in order to control the concentration and amount of fuel in the mixing tank. These auxiliary devices occupy a large volume thereby increasing the size of the apparatus. Such a conventional system is too large to be used for the power supply for a portable small appliance such as a mobile phone, portable audio unit, and notebook personal computer.
On the other hand, if the water tank and water recovery circulation path are removed to decrease the size of the fuel cell system, the fuel usage efficiency drops.